New antitumor agents, as well as clinically established drugs, are being evaluated for embryotoxic, cytogenetic and carcinogenic effects. A comparative study of the embryotoxic and cytogenetic effects in mice of a series of plant-derived antitumor agents (vincristine, maytansine, colchicine, VP 16-213 and VM-26) has been completed, as have similar studies on two adenosine deaminase inhibitors (2'-deoxycoformycin and EHNA). Studies have been initiated on the embryotoxic and cytogenetic effects of two inhibitors of de novo pyridmide nucleotide biosynthesis (N-(phosphonacetyl)-L-aspartic acid and 6-azauridine) and several anthracenediones. The carcinogenic activity of various clinically effective antitumor and/or immunosuppressive agents, including procarbazine, N-methylnitrosourea, melphalan, azathioprine and adriamycin, is under evaluation in long-term studies in monkeys. A survey of second malignancies developing in treated cancer patients, initiated several years ago, is continuously being updated using data from published reports and from several centers and hospitals.